Artificial butter flavoring is a potential etiological agent of obliterative bronchiolitis (OB) in employees at microwave popcorn packaging plants;however, the specific toxic component(s),and the mechanisms of toxicity are unknown. We evaluated the respiratory toxicity of diacetyl, the major volatile component of artificial butter flavoring. Subacute exposure to 200 or 400 ppm diacetyl for 5 days caused deaths, necrotizing rhinitis and inflammation of the large bronchi. Reducing the exposure to 1 hr/day (100, 200, 400 ppm) for 4 weeks caused less nasal toxicity, chronic bronchitis and laryngitis, without bronchiolar lesions. Lymphocytic bronchitis and mild nasal injury were observed after exposures to 1200 ppm (15 minutes, 2x/day, 4 weeks). Subchronic exposures to 100 ppm (6 hr/day, 12 wks) caused lymphocytic bronchitis and bronchiolitis with minimal necrosis in the nasal cavity. Treatment with 400 mg/kg by oropharyngeal aspiration to bypass the nose caused foci of fibrosis with little or no inflammation at the junction of the terminal bronchiole and alveolar duct. Collectively, these results indicate that occupationally relevant diacetyl exposures result in a pattern of injury that replicates features of OB. Similar studies are being conducted in the rat. Because of conerns about diacetyl toxicity, it is being replaced in some consumer products by 2,3-pentanedione, a structurally-related and untested chemical. In initial inhalation studies with rats and mice, 2,3-pentanedione was also found to be a pulmonary toxicant. In addition to injuring the epithelium lining the respiratory tract, 2,3-pentanedione also caused OB-like fibrotic lesions in the airways of rats.